1. Field of the Invention
The present invention relates to a device for the detection of passing blades in turbo-machinery.
2. Description of the Related Art
The detection of passing turbo machinery blade length (or tip clearance) and time of arrival is commonly performed using light probes, capacitive sensors, and eddy current sensors.
Passive eddy current sensors generally use permanent magnets and behave like small electrical generators. Relatively minute quantities of power are drawn from the spinning turbo machinery, so there is no need for an external power source specifically for the sensor. The magnetic reluctance of non-ferrous alloys commonly found in engine case walls, such as titanium and nickel is typically very close to that of air. Hence, there is no need to drill through-holes in the case wall, as with other types of sensors. The eddy current sensor described as the present invention sees through the wall, and many other types of debris, such as unburned fuel, ice, oil, or water, which may also be in the vicinity.
Passive eddy current blade sensing devices have been designed with one or more coils wound directly around one or more magnets, as Langley discloses in his 1989 patent “Eddy Current Clearance Transducing System” (U.S. Pat. No. 4,847,556) and again in his 1990 patent “Eddy Current Turbomachinery Blade Timing System” (U.S. Pat. No. 4,967,153). Langley describes an alternative design with the placement of a coil between two magnets. L. A. Rosenthal discloses a “Metal Detector” as a coil wound around a magnet in his 1962 patent (U.S. Pat. No. 3,065,412). However, the signal generated by these techniques usually includes complex features, such as double pulses, as depicted in FIG. 2 of the present application. The complex features of the pulse make it difficult to reliably measure tip clearance and time of arrival. A trigger threshold may be set at a value that results in a sporadic time of arrival reading that alternates between the leading peak and the trailing peak. The signal amplitude of a passive eddy current sensor usually varies with RPM and gap, hence triggering will occur on different parts of the blade at different speeds. Conversely, a signal processor easily and reliably triggers on the simple pulse generated by the present invention. This preferred pulse shape is depicted in FIG. 3 of the present application.
Several types of active eddy current sensors are currently available with two or more coils. One coil is driven with a time varying excitation voltage in the 1 kHz–1 MHz range. A second coil monitors changes in the magnetic field when electrical conductors are in the vicinity. There is no need for a permanent magnet, and one can use this type of sensor on a smooth shaft (no need for passing blades). However, the electronics and signal processors are much more complicated for this type of device, and the excitation frequency must be sufficiently high to detect the turbo-machinery blades passing in the 10–100 kHz range. GDATS and Microepsilon are two examples of active eddy current sensors with excitation frequencies sufficiently fast to detect passing turbo-machinery blades at normal operating speeds.
Rozelle et al. describes a system for monitoring shrouded blades in his 1992 patent “Shrouded Turbine Blade Vibration monitor and Target Therefor” (U.S. Pat. No. 5,097,711). However Rozelle's only references to the specific sensing devices cite the Bentley Nevada Proximitor and the “Self generating permanent magnetic sensors (e.g. Airpax sensors).” Bentley Proximitors have an excitation voltage and a frequency response of only 12 kHz. They are too slow to detect many modern turbomachinery blades, which pass a stationary sensor at up to 100 kHz. Airpax sensors have a wire coil wound on a ferro magnetic core, but the coil is placed between the magnet and the passing blades (ferrous gear teeth in most cases). This configuration is convenient for detecting the passage of ferromagnetic objects passing by the probe tip, however the magnet is placed too far from the passing blades for it to be of practical use with non-ferromagnetic blades commonly found in turbomachinery. Furthermore, the weak signal produced would have a double peak as in Rosenthal.
Rickman Jr. describes a Motion Sensor Utilizing Eddy Currents for non-ferrous blades through non-ferrous case wall materials (U.S. Pat. No. 4,439,728). However, his design calls for a relatively large magnet placed far from the sensing coil. This may be useful as a tachometer, however advanced blade vibration monitors will require a more compact probe capable of more precisely determining blade deflection relative to a single fixed point on the engine case wall. The present invention provides this feature by placing the bias magnet and pickup coil in the same barrel at the same mounting location.